Polychromatic light sources with independent intensity controls can offer the ability to generate any colour within the colour gamut of the constituent light sources. An example is a solid-state light fixture with red, green, and blue (RGB) light emitting diodes. There are a number of control systems that enable the control of a luminaire incorporating a plurality of different emission colour light sources. These control systems provide a means for the manipulation of the resultant blended illumination based on a desired illumination level.
For example U.S. Pat. No. 5,350,977 discloses a luminaire of variable colour temperature which includes a plurality of light sources of different emission colours which are lighted by a lighting means. The emission colours of the respective light sources are blended for emission of a blended colour light from the luminaire. The control means transmits a colour temperature control signal to the lighting means for varying the manner in which the emission colours are blended. The signal transmission from the control means to the lighting means is carried out such that the respective differences in the reciprocal colour temperatures (mireks) of the two adjacent stages of the colour temperature control signals are substantially equalised.
A method of automatically measuring the spectral content of a LED light source and controlling the spectral content based on that measurement with reference to a desired spectral content is disclosed in U.S. Pat. No. 6,448,550. The spectral content of a solid state illumination source composed of LED sources of different colours is measured by photosensors mounted in close proximity to the sources. The results of these measurements are used to control the spectral content of the blended light by varying the current to the different colour LEDs. The photosensors associated with the system can collect mixed light thus each colour of LED need not be measured separately. A desired spectral content is selected and the actual spectral content can be measured and adjusted to match the desired levels.
U.S. Pat. No. 6,127,7783 discloses a system where the combined light output or chromaticity, of a white light emitting LED luminaire is electronically controlled based on measurements by a single photodiode arranged to measure the light outputs of all the LEDs in the array. The light output of the LEDs in each colour is separately measured using a sequence of time pulses. During each time pulse, the current for the colours not being measured is turned off. The response time of a photodiode is short enough that the measurement can be taken in less time than what can be observed by the human eye. The measured light outputs for the colours are compared to desired outputs, which may be set by user controls, and changes to the power supply for the colour blocks are made as necessary.
U.S. Pat. No. 6,507,159 discloses a control system for an RGB LED luminaire that compares the feedback tristimulus values representing the mixed light output of an RGB LED luminaire with the referenced tristimulus values representative of the desired emitted light. The control system adjusts the forward currents of the LED luminaire such that the difference between these tristimulus values is decreased to zero. Particularly, the controlling system comprises a feedback unit including photodiodes for generating the feedback tristimulus values of the LED luminaire, and a controller for acquiring the difference between the feedback tristimulus values and the desired reference tristimulus values. The controller generates control voltages for adjusting the forward currents of the LED luminaire so that the difference between these tristimulus values is decreased to zero. The tristimulus values under comparison may be under the CIE 1931 tristimulus system or under a new RGB colourimetric system. Under a steady state where the feedback tristimulus values follow the desired reference values, the light produced by the RGB LED luminaire has the desired target colour temperature and lumen output, which can be regulated to the targets regardless of the variations in junction temperature, forward current and ageing of the LEDs in the RGB LED luminaire.
An RGB LED controller system that employs a feedback control arrangement that substantially corrects all colour point errors without visual perception of change in colour is disclosed in U.S. Pat. No. 6,552,495. This control system comprises a sensor responsive to light generated by the LEDs to measure the colour co-ordinates of the generated light, wherein the colour co-ordinates are defined in a CIE(x, y, z) colour space. A transformation module is connected to the sensor to transform the co-ordinates of the generated light to a second colour space, such as an (x′, y′) colour space, in accordance with a Farnsworth transformation. A reference module is configured to provide reference colour co-ordinates corresponding to the desired light, expressed in the second colour space. An error module is coupled to the transformation module and the reference module and this error module are configured to generate an error colour co-ordinate corresponding to the difference between the desired white light colour co-ordinates and the generated white light colour co-ordinates. A driver module is coupled to the error module and is configured to generate a drive signal for driving the LEDs in response to this difference.
U.S. Pat. No. 6,441,558 discloses a controllable white LED luminaire using red, green and blue LEDs. A light control system is configured to maintain the colour temperature and the lumen output level of the emitted white light. The control system comprises a feed-forward temperature compensation arrangement and an optical feedback control system to maintain the target white light. The junction temperature and the light output of the LEDs are sensed and are fed into the light control system. The temperature feed-forward compensation system corrects the deviation in the target colour temperature and the colour-rendering index of the white light. A processing means, such as a feed-forward temperature compensator means, is configured to provide required lumen output fractions of the RBG light sources in response to the junction temperature of the LEDs and the target white light. A lumen output model in combination with a lumen output controller are configured to maintain the light output generated from the LED light source equal to the light output value provided by the feed-forward temperature compensator, regardless of junction temperature, ageing and batch-to-batch variation of the LEDs.
The luminous intensity of LEDs is however dependent on their spectral distribution, junction temperature, drive current, non-linear luminous flux output characteristics, peak wavelength shifting and spectral broadening characteristics, device ageing and manufacturing tolerances which include for example binning for peak wavelength, luminous intensity and forward voltage. As such a successful design of a control system for such a lighting system would include optical feedback from a sensor that monitors both colour and intensity as is outlined in the prior art. However, this scenario introduces additional design issues such as variations in colour sensor spectral responsivities, sampling rates, and feedback loop response times. In addition, approximations are introduced by linear colour spaces when translating the sensor signals into a model of human colour vision, for the perceived illumination. A typical approach to such a problem is to implement a proportional integral-derivative (PID) controller whose feedback control signal is a weighted sum of the instantaneous error, the integral of the error, and the derivative of the error, wherein this approach implicitly assumes that the process being controlled is linear. The combination of junction temperature dependencies, square law dimming, and colour space mapping may therefore preclude the effective use of linear PID controllers as is done in the prior art. As such there is a need for a control system for controlling a plurality of discrete light sources having varying peak wavelengths of emissions that can account for these non-linear factors, thereby providing the dynamic control of the lighting system.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.